Spinning on Earth

I’m always happy to entertain things outside of the normal news. So I’ve been saving a question until it seemed like a good time to bring it up. Today is a good time, since so many people around me tell me they’re stressed out by the news these days. Let’s just relax then, and get our science on.

Here’s the question that came my way a couple of months ago: Since Earth is constantly rotating, how can astronomical telescopes stay aimed at their target objects? In other words, wouldn’t an Earthbound telescope have to be constantly adjusted in order to keep its aim?

Yes, indeed. A scope does have to be constantly adjusted to counter Earth’s rotation.

Many telescopes automatically slew themselves with electric motors to maintain their point of aim. I probably haven’t seen all the methods for this, but I can mention a few that I have seen.

At a rough level this can be accomplished merely by a motor and geometry. One common way of addressing the geometry is with an “equatorial mount,” which allows the scope to be slewed across an arc. Not just any arc, mind you, but an arc that counteracts the relative motion of Earth’s rotation.

Other solutions use electronics for more precise guidance. Instead of being driven blindly by a motor that relies on pure geometry, the motorized telescope mount takes feedback from an optical sensor (sort of like an electric eye) to track and keep on top of the targeted object. Various types of mounts use electronic guidance, and some of this stuff is very sophisticated.

This is a big deal in photography, where long exposures can be necessary to gather up as much light as possible from faint objects. The scope has to maintain its accurate aim for a long time. These exposures are sometimes on the order of hours. They are typically taken as a series of shorter exposures and then electronically “stacked” using specialized software.

Sometimes, though, simple measures work for certain situations. If an object is really bright, like the moon, one of the juicier planets, or maybe a showy comet, then even a smartphone held up to the telescope can take a cool shot.

A normal snapshot without a scope can do the trick in some cases. For example, a Saipan Tribune reader sent me a shot he took of the “Beaver Moon” on Saipan last November. I really enjoyed getting that photo.

Motors and electronics aren’t the only way to keep a scope aimed. So now we’ll consider some manual approaches. These aren’t precise methods, at least by modern standards, but they’re good enough to keep some cosmic blob within the rough confines of the eyepiece.

This leads us to something called a German Equatorial Mount, which is common even in some very low-priced scopes. Some have a manual knob instead of a motor. The user turns the knob to keep the scope aimed. For larger movements, such as pointing the scope at a new object in the sky, other knobs are turned. The whole thing is a lot of rigmarole.

Another manual method is to aim a scope like you’d aim a video camera that’s atop a tripod. This is common for small scopes. But for anything larger, it doesn’t seem very popular for manual guidance.

There is a common design that simply allows the user to just grab the doggone scope and point it. It works with small scopes and with large (by amateur standards) scopes as well. This is called the Dobsonian design, named for John Dobson, who passed away in 2014 at age 98. This guy is a legend. His efforts really brought scopes to the masses.

A Dobsonian scope operates by the same principle that pirates use for adjusting cannon fire. Like a cannon barrel, the tube of the telescope sits in a wooden box, in which the tube can pivot up (higher elevation) or pivot down (lower elevation). Meanwhile, the wooden box itself sits atop a lazy Susan, the same sort of spinning, tabletop disk you see at Chinese restaurants so the various dishes can be rotated among the people sitting around the table. This principle allows for the left- and right movement of the scope.

The Dobsonian design is simple, cheap, and well-suited for manual guidance. It makes astronomy as easy as shooting spitballs. This is the flavor I use. I’m always nudging the scope ever so slightly to keep things centered in the eyepiece as Earth’s rotation conspires to drag the object out of view.

There are, incidentally, various electronic ways to slew Dobsonians, but I have not put my hands on any of these examples.

Anyway, those are a few highlights of how scopes are pointed so that our spinning planet doesn’t mess things up.

I’m already daydreaming about taking an astronomy trip next month. It sure beats reading the news. Hey, dreaming won’t kill you. But stress sure will.